This invention relates particularly to salient-pole shaded pole motors having concentrated windings disposed around circumferentially spaced apart pole pieces that each have a leading pole tip separated by an air gap from the trailing pole tip of the pole piece adjacent thereto. Shaded pole motors of this type are one of the less expensive types of motors to manufacture. Thus, this motor type is usually selected for alternating voltage applications whenever the heretofore known operating characteristics (in terms of starting torque, maximum or break down torque, running torque, dip torque, efficiency, etc.) of this motor type will meet the needs of an intended application.
If one or more operating characteristics of this type of motor is not satisfactory for a given application, however, distributed wound motors will normally be used. Generally speaking, distributed wound motors are more expensive to manufacture than shaded pole motors. In addition, mechanical duty distributed wound motors utilize approaches such as selectively energizeable auxiliary winding arrangements so that desired operating characteristics can be achieved. For example, increased starting or locked rotor torque, efficiency, and so forth can be realized with these more expensive motors as compared to prior salient-pole shaded pole motors of similar overall physical size.
Shaded pole motors of the concentrated winding salient-pole variety, when designed to be relatively efficient during operation (e.g., those having efficiencies of 35% to 40% and greater) have relatively low starting torques. For example, when motors of this type have an operating efficiency in the neighborhood of 40% or more, the ratio of starting or locked rotor torque to maximum torque seems invariably to be about 0.33 or less. This is one of the primary reasons why the use of shaded pole motors has been generally limited to applications for driving fans and other fluid moving devices such as pumps. In many of these applications, the needed locked rotor torque is a relatively small fraction of the desired maximum torque or running torque (expressed as a percentage of maximum torque); although in some specific fan applications, the locked rotor torque may have to be in the neighborhood of one-half of the rated running torque.
In the more efficiently designed concentrated winding salient-pole shaded pole motors of which I am aware, pole pieces project radially from a magnetizeable yoke. In addition, the pole tips of adjacent ones of such pole pieces are spaced apart by air gaps (as shown, for example, in Arnold U.S. Pat. No. 3,313,965) and thus are not interconnected with magnetic material. With designs of this general type, changes that increase efficiency (for a given dip to maximum torque ratio) will decrease the ratio of locked rotor torque to maximum torque. On the other hand, for a given locked rotor torque and maximum torque, any further improvement in efficiencies causes, expectedly, a decrease in dip torque (DT).
While a reduction in dip torque may be generally undesirable, it may become intolerable (because of loss of motor stability) in motors designed with tapped windings and intended for multispeed operation. For example, while a multispeed salient-pole shaded pole motor may be stable for high speed fan operation; when the motor is energized for low speed operation, it will not come up to speed if the dip torque is less than the amount of torque needed to accelerate the fan or other load past the speed associated with the dip torque of the motor. However, for a given locked rotor torque and maximum torque, any increase in stability associated with increased dip torque causes a reduction in operating efficiency with prior art approaches.
It therefore should now be understood that it would be advantageous and desirable to provide new and improved salient-pole shaded pole motors having winding coils concentrated about radially disposed pole pieces; such motors having characteristics that would not make it necessary (among other things) to sacrifice efficiency for increased locked rotor torque to maximum torque ratios for a given dip torque to maximum torque ratio. It would also be of importance to provide new and improved salient-pole shaded pole motors with characteristics that would permit the use of this type motor in so-called mechanical duty applications where the more expensive types of induction motors (with auxiliary starting devices) have been used heretofore. Two general examples of this type of application is the business machine field and electric motor driven gear reducer fields. It also would be desirable to provide a way to increase the efficiency of small fan motors and specially with minimum increase in cost.
Accordingly, it is an object of the present invention to provide new and improved concentrated winding salient-pole shaded pole motors that have higher efficiencies than were known heretofore, and that have greatly improved locked rotor torque characteristics, and yet with minimum cost increases.
It is another object of the present invention to provide methods of making motors of the just mentioned type wherein the inter-relationships between various characteristics such as dip torque, locked rotor torque, maximum torque, and current or power requirements are basically different as compared to motors of the same type known heretofore.
Still another object of the present invention is to provide new and improved salient-pole shaded pole motors wherein the above and other objects may be fulfilled without necessarily making drastic increases in the physical size of motors of a given power rating or in the cost of manufacture.